Aim To assess temporal changes in gammarid distribution in Brittany and microhabitat-use overlap between the endangered endemic Gammarus duebeni celticus Stock &amp Pinkster, 1970, the expanding natives G. pulex (Linnaeus, 1758) and Echinogammarus berilloni (Catta, 1878), and the introduced G. tigrinus Sexton, 1939. Location Brittany and adjacent regions in western France. Methods The spatial and temporal patterns in distribution of gammarids at the scale of Brittany were studied using 351 sites. Longitudinal distributions (from the source to the estuary of the river) and microhabitat-use (substratum type and water velocity) were also considered in selected rivers. Results At the regional scale, all species occurred together less often than expected statistically, with significant deviations from expected for G. pulex vs. both G. duebeni celticus and G. tigrinus, and for E. berilloni vs. both G. duebeni celticus and G. tigrinus. However, at the microhabitat scale, E. berilloni occurred significantly more often than expected with the endemic G. duebeni celticus, and this appears to be due to similar substratum and water velocity preferences, although at both the regional and microhabitat scales E. berilloni prefers wider streams than G. duebeni celticus. This study reveals a decline in the endangered G. duebeni celticus since 1970. Main conclusions The longitudinal and local distributions of G. duebeni celticus, and the higher-than-expected co-occurrence of the species with G. pulex, suggest that the decline of the endemic species may be due to changes in the environment and/or interference from native G. pulex, which is expanding its range in Brittany. The results are discussed as regards to the consequences for regional biodiversity.

1. Invasive species in aquatic systems are major drivers of changes in biodiversity. Amphipods are key species in freshwaters, with invasive amphipods either replacing or coexisting with native species and often damaging local biodiversity. However, the consequences of interactions among native and invasive amphipods for their habitat use and feeding ecology and ecosystem function are not yet well understood. 2. We examined a number of streams in Brittany and Northern Ireland, with native and invasive amphipods, to evaluate the consequences of species interactions for both habitat use and diet. Our field studies centred on testing two proposed models: a cohabitation model without competition between two native species (Gammarus pulex vs Echinogamm- arus berilloni), and a competition model between an invasive and a native species (Gammarus pulex vs Gammarus duebeni celticus). For these three species, alone and in combination, we assessed their habitat use and feeding patterns, the latter through gut contents and stable C and N isotope analyses of their tissues. 3. When existing as single-species populations, all three species used stream habitats broadly similarly, although G. pulex was more strongly associated with leaf litter and vegetation compared to pebble substrata than the other species. When G. pulex coexisted with either E. berilloni or G. d. celticus, the latter two changed to using all habitats equally, whereas the former retained its habitat preferences. 4. Similarly, all three species when alone had similar gut contents, with inorganic material predominating, followed by leaf and woody material and more rarely algae and invertebrates. When G. pulex coexisted with E. berilloni, the diet of the latter did not change; however, the frequency of inorganic matter, leaves and wood declined in the gut contents of G. pulex. When G. pulex coexisted with G. d. celticus, the pattern of gut contents did not change in either species. 5. When existing as single-species populations, G. pulex had a broader range of isotopic signatures, both for d13C and for d15N, than the two other species, indicating a more variable diet among individuals. When G. pulex coexisted with either E. berilloni or G. d. celticus, the latter two had similar ranges of d13C and d15N, whereas for G. pulex the range was much less for d13C and d15N, suggesting a less diverse diet. 6. Our results infer two different modes of coexistence between native and non-native amphipods. We have shown that the native species, which coexist stably, appear to show interference competition, leading to spatial habitat segregation, whereas competition for food and possible intraguild predation by G. pulex on G. d. celticus would explain why the distribution and density of the latter is affected by G. pulex. However, since all the species have a similar diet and feeding habit, we expect no great overall effect on ecosystem processes as a consequence of species interactions and displacements.